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Journal of Asian Earth Sciences 30 (2007) 375–389 www.elsevier.com/locate/jaes 1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2006.11.005 Tectonic controls on the late Miocene–Holocene volcanic eruptions of the Tengchong volcanic Weld along the southeastern margin of the Tibetan plateau Yu Wang a,¤ , Xuemin Zhang b , Chaosong Jiang c , Haiquan Wei d , Jinglin Wan d a Geologic Laboratories Center and Department of Geology, China University of Geosciences, Beijing 100083, China b Institute of Geology, China Seismological Administration, Beijing 100029, China c Seismological Bureau of Yunnan Province, Kunmin 540000, China d Institute of Geology, China Seismological Administration, Beijing 100029, China Received 23 January 2006; received in revised form 7 June 2006; accepted 6 November 2006 Abstract The Tengchong volcanic Weld, located along the southeastern margin of the Tibetan plateau, experienced multiple eruption stages since the late Miocene, including time intervals of »5.5–4.0 Ma, 3.9–0.9 Ma, 0.8–0.01 Ma, and younger than 0.01 Ma. These eruption stages produced diVerent volcanic rocks, principally basaltic and basaltic-andesite series. At the same time or prior to volcanic eruptions in the Tengchong volcanic Weld, NE–NNE-trending rift basins and NS-striking normal faults formed during the late Miocene–Pliocene. In addition, rapid exhumation of the neighboring mountains occurred at »6–5 Ma constrained by apatite Wssion track dating and its ther- mochronological modeling. At present, the state of stress in the Tengchong volcanic Weld and its surroundings is NNE–NE-compression and WNW–NW-extension based on seismic foci mechanisms. Petrologic and geochemical data indicate that the source of the Tengchong volcanic rocks belongs to an intracontinental tectonic setting, but not a subduction or collision zone between Indian and Eurasian plates. Since the late Miocene, the dextral strike-slip motion of the Sagaing fault induced E–W-extension. The Sagaing dextral strike-slip motion might disturb the lower crust-upper mantle of the Tengchong block, resulting in the partial melting of the upper mantle which, in turn, induced volcanic eruptions characterized by mature island-arc features. © 2006 Elsevier Ltd. All rights reserved. Keywords: Tengchong volcanic Weld; Pliocene–Holocene; Tectonic setting; Subduction and collision; Sagaing fault; Dextral strike-slip motion 1. Introduction Widespread volcanic eruptions occurred in the Tibetan plateau and its adjacent areas in Cenozoic time, including along the Yalung–Zangpo suture zone, northern Tibet and southeastern margin of the Tibetan plateau (Le Dain et al., 1984; Stephenson and Marshall, 1984; Whitford- Stark, 1987; Xizang Bureau of Geology and Mineral Resources, 1993; Cong et al., 1994; Wang, 1999; Wei et al., 2003). The late Miocene-Quaternary volcanic eruptions are distributed along the Gangdese belt, north of the Yalung-Zangpo suture zone, the Kunlun Mountains belt, the Burma arc, and the Tengchong area in Yunnan Prov- ince of China (Fig. 1). The Tengchong volcanic Weld is located along the southeastern margin of the Tibetan pla- teau, at 24°40–25°30N, 98°15–98°45E, and since the late Miocene time, volcanic eruptions have discontinu- ously occurred. At present, numerous active hot springs along or around the volcanic clusters indicate sites of active geothermal Welds (Wang and Huangfu, 2004) and the potential for future eruptions. Petrologic, geochemical, and regional tectonic data has lead to diVerent interpretations of the tectonic setting of the Tengchong volcanic Weld. Zhu et al. (1983) and Mu et al. (1987) argued that the Tengchong volcanic eruptions * Corresponding author. Tel.: +8610 82321028; fax: +8610 82321006. E-mail address: [email protected] (Y. Wang).
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Page 1: Tectonic controls on the late Miocene–Holocene … Y. et...Since the late Miocene, the dextral strike-slip motion of the Sagaing fault induced E–W-extension. The Sagaing dextral

Journal of Asian Earth Sciences 30 (2007) 375–389

www.elsevier.com/locate/jaes

Tectonic controls on the late Miocene–Holocene volcanic eruptions of the Tengchong volcanic Weld along the southeastern

margin of the Tibetan plateau

Yu Wang a,¤, Xuemin Zhang b, Chaosong Jiang c, Haiquan Wei d, Jinglin Wan d

a Geologic Laboratories Center and Department of Geology, China University of Geosciences, Beijing 100083, Chinab Institute of Geology, China Seismological Administration, Beijing 100029, China

c Seismological Bureau of Yunnan Province, Kunmin 540000, Chinad Institute of Geology, China Seismological Administration, Beijing 100029, China

Received 23 January 2006; received in revised form 7 June 2006; accepted 6 November 2006

Abstract

The Tengchong volcanic Weld, located along the southeastern margin of the Tibetan plateau, experienced multiple eruption stagessince the late Miocene, including time intervals of »5.5–4.0 Ma, 3.9–0.9 Ma, 0.8–0.01 Ma, and younger than 0.01 Ma. These eruption stagesproduced diVerent volcanic rocks, principally basaltic and basaltic-andesite series. At the same time or prior to volcanic eruptions in theTengchong volcanic Weld, NE–NNE-trending rift basins and NS-striking normal faults formed during the late Miocene–Pliocene. Inaddition, rapid exhumation of the neighboring mountains occurred at »6–5 Ma constrained by apatite Wssion track dating and its ther-mochronological modeling. At present, the state of stress in the Tengchong volcanic Weld and its surroundings is NNE–NE-compressionand WNW–NW-extension based on seismic foci mechanisms. Petrologic and geochemical data indicate that the source of the Tengchongvolcanic rocks belongs to an intracontinental tectonic setting, but not a subduction or collision zone between Indian and Eurasian plates.Since the late Miocene, the dextral strike-slip motion of the Sagaing fault induced E–W-extension. The Sagaing dextral strike-slip motionmight disturb the lower crust-upper mantle of the Tengchong block, resulting in the partial melting of the upper mantle which, in turn,induced volcanic eruptions characterized by mature island-arc features.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Tengchong volcanic Weld; Pliocene–Holocene; Tectonic setting; Subduction and collision; Sagaing fault; Dextral strike-slip motion

1. Introduction

Widespread volcanic eruptions occurred in the Tibetanplateau and its adjacent areas in Cenozoic time, includingalong the Yalung–Zangpo suture zone, northern Tibetand southeastern margin of the Tibetan plateau (Le Dainet al., 1984; Stephenson and Marshall, 1984; Whitford-Stark, 1987; Xizang Bureau of Geology and MineralResources, 1993; Cong et al., 1994; Wang, 1999; Wei et al.,2003). The late Miocene-Quaternary volcanic eruptionsare distributed along the Gangdese belt, north of the

* Corresponding author. Tel.: +8610 82321028; fax: +8610 82321006.E-mail address: [email protected] (Y. Wang).

1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2006.11.005

Yalung-Zangpo suture zone, the Kunlun Mountains belt,the Burma arc, and the Tengchong area in Yunnan Prov-ince of China (Fig. 1). The Tengchong volcanic Weld islocated along the southeastern margin of the Tibetan pla-teau, at 24°40�–25°30�N, 98°15�–98°45�E, and since thelate Miocene time, volcanic eruptions have discontinu-ously occurred. At present, numerous active hot springsalong or around the volcanic clusters indicate sites ofactive geothermal Welds (Wang and Huangfu, 2004) andthe potential for future eruptions.

Petrologic, geochemical, and regional tectonic data haslead to diVerent interpretations of the tectonic setting ofthe Tengchong volcanic Weld. Zhu et al. (1983) and Muet al. (1987) argued that the Tengchong volcanic eruptions

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376 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

were derived from the subduction–collision zone betweenIndian and Eurasian plates. Zhao and Chen (1992)believed that it was a post-collision arc-volcanism or adelayed arc-volcanism in the southeast margin of theTibetan plateau. Cong et al. (1994) proposed that theTengchong volcanic swarms were derived from a matureisland-arc area. Ji (1998) pointed out that during 5–8 Ma,the lower lithosphere delaminated in the Tengchong area,resulting in upwelling of the upper mantle. In contrast,Chen et al. (2002) proposed that the magma source forthese volcanic rocks was enriched-mantle.

These interpretations are mutually contradictory.Although the timing of volcanism in the Tengchong vol-canic Weld was same as shoshonite eruption west ofthe Kunlun Mountains belt in the northern Tibetanplateau (Wang, 1999) and overlapping in age with erup-tions of Burma volcanoes along the Sagaing fault, thetectonic setting of the Tengchong volcanic Weld is notclear. Is it related to the Sagaing dextral strikes-lip fault,the subduction–collision tectonics between Indian andEurasian plates, or some intracontinental tectonic set-ting?

Based on structural mapping (1:50,000), basin sedimen-tary analysis, K–Ar data, geochemical data of volcaniceruptions, eruptions sequences of volcanic rocks, seismicfoci mechanisms, and apatite Wssion-track dating and itsthermochronological modeling, we analyzed sequences ofthe volcanic eruptions and characterized the relationsbetween volcanic eruptions and structural development ofthe region. Also, we attempt to reconstruct the tectonicsetting of volcanic eruptions for the Tengchong volcanicWeld since the late Miocene–Pliocene.

2. Geological background and features of the Tengchong volcanic Weld

2.1. Geological background

The tectonic cross-section from west to east, displays aseries faults or suture zones, they are: Naga Hills subduc-tion zone, Sagaing dextral fault, Myitkyina suture zone,Burma gneissic belt, Yingjiang island-arc magmatic belt,Tengchong volcanic area, Gaoligong metamorphic belt,Baoshan Block, Red-River fault zone, and Yangtze Plate(Fig. 1). The Tengchong volcanic Weld is located on thewestern side of the uplifted Gaoligong metamorphic belt.Along the western side of the Tengchong area, late Meso-zoic to early Cenozoic island-arc granites and gneiss areexposed between Burma and southwestern China (YunnanBureau of Geology and Mineral Resources, 1979; Luo andHu, 1983; Ji, 1998; Jiang, 1998). The Tengchong volcanicWeld is located within a N–S-trending fault zone. In thisarea, island-arc granites intruded into Mesozoic and Ceno-zoic geologic units which represent the subduction of theIndian plate under the Eurasian plate along Naga Hills andMyitkyina-Mandalay suture zones (Ji, 1998). Sinistralstrike-slip motion accommodated southeast escape of theTibetan plateau between 27 and 15 Ma (Molnar and Tap-ponnier, 1978; Tapponnier et al., 1982; Zhong, 1997; Wangand BurchWel, 1997; Ji, 1998; Socquet and Pubellier, 2005).

2.2. Geophysical features

The spatial distribution of earthquakes during 1998–1999 shows that, earthquakes are mostly shallower than

Fig. 1. Tectonic site of the Tengchong volcanic Weld along the southeastern margin of the Tibetan plateau. Location of Fig. 2 and sample YN3 are shown.

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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389 377

15 km beneath the Tengchong area, but at depths of»40 km to its west or east (China Earthquake Catalog).The chromatography of this area shows that there are ahigh velocity body at 10–15 km depth, a low velocity bodyat 16–24 km and a high velocity body at 25–40 km (Qinet al., 2000a; Wang et al., 2002; Wang and Huangfu, 2004).The low velocity body is proposed to be a magma body orpartly-molten material (Qin et al., 2000b). In addition, thedepth to the top of the high conducting layer in the uppermantle is shallow at the Tengchong volcanic Weld, onlyabout »64 km in depth (Sun et al., 1989). But at only»50 km deep to the west, it changes abruptly on the bothsides of the Gaoligong mountains belt.

Wu et al. (2001) used receiver function to calculate thevelocity structure of the Yunnan Province. Their resultsshow that the thickness of the crust reduces graduallyfrom northwest to southeast, whose shape and scale coin-cide with Sichuan-Yunnan relatively rigid blocks. Mohosurface beneath the Tengchong area is about »38 kmdeep, and to its east of the Baoshan block, the crustalthickness is »40–45 km. Wang et al. (2002) studied thevelocity of Sichuan and Yunnan areas with tomography,and determined that, compared with their surroundings,the Tengchong volcanic Weld has not only a negativeanomaly in the uppermost mantle, but also a negativeanomaly in the upper crust and a positive anomaly in thelower crust. Unlike its northwest, the Tengchong volcanicWeld has no additional evidence of crustal thickening.

2.3. Types of volcanic rocks and geochemical characteristics

In the Tengchong volcanic Weld, major volcanic rocksare basalt, dacite welded tuV, basaltic trachyandesite andtrachyandesite. All of them belong to a high-potassiumcalc-alkaline volcanic suite. Volcanic eruptions in thisregion can be roughly divided into at least four stages orswarms: late Miocene–Pliocene basalt and olivine-basaltvolcanic rocks (5.5–4.0 and 3.8–0.9 Ma) and Pleistoceneacid rocks (0.8–0.1 Ma); late Pleistocene–Holocene basaltsand intermediate-acid rocks such as andesites (0.1–0.01 Ma) (Mu et al., 1987; Ji, 1998; Li et al., 1999; Wanget al., 1999).

From Pliocene–Pleistocene to Holocene, K2O contentincreases in the volcanic rocks from 1.5% to 3.65%, butMgO content decreases from 5.91% to 3.04% (Fan et al.,1999). The Tengchong volcanic component has highAl2O3 and K2O, but low TiO2 and LREE, similar toisland-arc volcanic rocks. On the diagram of K2O–SiO2, itbelongs to high-potassium basalt and andesite, as well, inthe Log �–Log � diagram, it drops into the island-arcdomain (Zhao and Chen, 1992). Nd–Sr isotopic andmicro-element analysis indicates that the main-seriesrocks are sourced from metasomatic mantle eclogite andpyrolite (Zhu et al., 1983). Basalts and andesitic basaltsare characterized by high 87Sr/86Sr ratio (0.7057–0.7081),low �Nd values (¡1.1 to ¡5.7), and particularly high208Pb/206Pb ratios (1.08–1.12) (Chen et al., 2002).

3. Constraining structures of the Tengchong volcanic Weld

On the eastern side of the Tengchong volcanic Weld, theGaoligong mountains belt was rapidly uplifted andexhumed to its present elevations in the time interval of»6–5 Ma, meanwhile, west-dipping and steep (>80°) nor-mal faults were developed. In the Tengchong volcanic Weld,the structural framework shows that its eastern and westernparts were uplifted, but the central region was depressedand the elevation diVerence between uplift and depressionis more than 500 m.

3.1. Structural framework of the Tengchong volcanic Weld and its evolution

In the Tengchong volcanic Weld, faults and volcanic clus-ters are distributed mainly within N–S, NE and NW trend-ing zones. A structural framework, built up by theextensional basins with NE–NNE trends together withtheir marginal faults, indicates an “arc”-type feature(Fig. 2). The north Xank of the “arc”-type structure trendsN330° and the south Xank trends NE40–50°. Some parts ofthe “arc”-type structure were covered with Pleistocene vol-canic rocks. NW–NNW-striking faults are characterized bysinistral slip and transpression while NE–NEE-strikingfaults are characterized by dextral slip and transtension(Fig. 3).

Based on the correlation of faults, folds, sedimentary anal-ysis and volcanic eruptions, three structural stages can be dis-tinguished. The shear-extension or transtension deformationstarted during late Miocene–Pliocene (»6–4 Ma). After theformation of rift basins, the Wrst stage volcanic eruptionsoccurred. After this event, during the Middle-Early Pleisto-cene, the NW–SE-direction extension resulted in normalfaults in this region; at the same time, eruption of the secondstage volcanic rocks occurred (0.8–0.1 Ma). Later, NNE–SSW-striking dextral strike-slip and normal faulting con-trolled the formation of latest three volcanic clusters: Hei-kongshan, Dayingshan and Maanshan volcanoes.

BrieXy, in the Wrst structural stage, structures are focusedon the southeastern part of the Tengchong volcanic Weldseen as the NE–SW-striking normal and dextral faults.During this stage, NE-striking extensional faults controlledextensional sedimentary basins. The third-stage faultingformed during middle-late Pleistocene in the central Teng-chong area, which controlled the distribution of the latestvolcanic eruptions. From the edges to the central part ofthe volcanic area, the faults were developed from the earlyPliocene to Pleistocene, but in the central part, faults wereactive from the Pleistocene to Holocene. During thesestructural stages, volcanic eruptions and faulting are closelyrelated spatially and temporally.

3.2. Basins and related sedimentation

During Miocene–Pliocene time a series of rift basinswere formed in the Tengchong and surrounding areas

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378 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

(Fig. 2). In the Tengchong Basin, the basement is volcanicrocks with ages of »5.5 Ma (ages based on K–Ar methoddating on volcanic rocks that crop out on the margins ofbasins; Ji, 1998). The basins are constrained by the NE- andNS-striking faults. The distributions of volcanic rocks anderuption clusters imply that the volcanism in the Teng-chong area has been related to local extensional stress sincethe Pliocene.

Within these extensional basins, Upper Miocene Namu-lin Formation and Pliocene Mangbang Formation weredeposited, and then, folded. After folding, depositionappears to have ceased. A couple of small oVset (<10 m)faults in the central region of the Tengchong volcanic Weld(i.e. the Liuhuangtang and Huangguaqing faults) are char-acterized by dextral strike-slip movement.

3.3. Distributions of the volcanic rocks and relationships to structures

Volcanic eruption centers migrate from the marginstowards center of the basins during the Pliocene–Pleisto-cene (Figs. 4 and 5). On the both sides of the NS-strikingfaults in the Tengchong volcanic Weld, the timing of volca-nic eruptions is older than that of eruptions along orwithin the NS-striking faults. Also, from south to northalong the volcanic zone, the age range of eruptions wasfrom »4 to »1.0 Ma (Mu et al., 1987). In its eastern andwestern parts, volcanic eruptions were earlier, but in thecentral part they were later. The youngest eruptions of theandesites and trachyandesites was <10,000 yrs (Wanget al., 1999).

Fig. 2. Distribution of late Cenozoic basins and “arc”-shaped structures surrounding the Tengchong volcanic Weld (modiWed from Yunnan Bureau ofGeology and Mineral Resources, 1979; Liao and Guo, 1986; Jiang et al., 1998). Abbreviations are as follows: F1, Nujiang fault; F2, Longchuanjiang-Lon-gling-Ruili fault; F3, Xiaolongchuan-Tengchong-Ruidian fault; F4, Lianghe-Guyong fault; F5, Wanding fault; B1, Jietou basin; B2, Luxi basin, B3,Husha basin, B4, Longchuan basin, B5, Zhefang basin. Figs. 3–5 are shown.

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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389 379

3.4. Rapid exhumation of the Gaoligong mountains belt on the eastern side of the Tengchong volcanic Weld

3.4.1. Apatite Wssion-track methodApatite was dated by the Wssion-track method. Single

apatite crystals were separated from 3 to 4 kg rock samplesusing conventional separation techniques. Approximately300–500 crystals of apatite were picked. Analytical proce-dures followed the external detector method described byGleadow and Duddy (1981) and were completed in theNeogeochronological Laboratory of Institute of Geology,China Seismological Administration. The 0.05–0.3 mm apa-tite were Wxed in epoxy and ground and polished to exposeinternal surfaces of the crystals, and then etched in 7%HNO3 at 20 °C for 35 s to reveal 238U spontaneous Wssiontracks. The surfaces of the crystals were then covered with awhite mica external detector. The crystals with an externaldetector were irradiated in the Heavy Water ResearchReactor (HWRR) in the China Institute of Atomic Energy.Induced tracks were revealed in muscovite external detec-tors by etching in 40% HF at 20 °C for 20 min. Grainmounting, polishing, and etching were all done by standardtechniques using the external detector method which aredocumented in Hurford and Green (1982). Fission tracksand track-length were counted and measured under an

OLYMPUS microscope using a magniWcation of £1000immersion objectives. Only those grains mounted in theplane of the C-axis for both mineral types were counted.International standard samples of Durango apatite(31.4 Ma) and Fish Canyon TuV apatite (27.8 Ma) wereused in the Zeta calibration, and SRM612 uranium glasswas used as the standard glass. All samples were counted byJ. L. Wan with zeta values of 352§19. Using the formulafrom Hurford and Green (1982), both pool and central agesof the apatites were calculated, in which the error associ-ated with this zeta factor has been propagated into the cal-culation of the grain and sample ages. Fission-trackapparent ages were determined for apatite from four sam-ples. The analytical data are listed in Table 1. The annealingtemperature for Wssion-track of apatite is estimated to beabout 110§10°C (Green et al., 1985; Hurford, 1986), andthe ages are given with an error of §1�. The single-grainage distributions for two samples passed a P(x2) testindicate a homogeneous age for each (Fig. 6).

3.4.2. Apatite Fission-track dating and track length-age modeling

Three samples from the Gaoligong mountains belt havesimilar apatite Wssion-track central ages in the range of5.2§ 0.4–6.4§ 0.7 Ma (Fig. 6, Table 1), but a sample from

Fig. 3. Structural diagram and distributions of latest volcanic eruptions in the Tengchong volcanic Weld.

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380 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

the Baoshan block on the eastern side of the Gaoligongmountains belt yields an apatite Wssion-track age of 44§3Ma. The distribution of track lengths yields a mean lengthof 12–14�m (Table 1).

Scattered ages from »5 to »44 Ma in samples from sim-ilar elevations but belonging to two tectonic units indicate along-lived evolution. To further interpret the signiWcance ofthe data, three apatite Wssion track data sets were modeledto quantify the timing and amount of cooling at speciWclocations. Modeling followed the approach of Ketchamet al. (1999, 2000). The results of thermal modeling of threesamples are presented in Fig. 7.

Apatite Wssion track ages, combined with modeled ther-mal histories from the Gaoligong mountains belt, reveal arapid cooling period since the end of Miocene to Pliocene,but diVers from cooling process on the eastern side of theGaoligong mountains belt. Samples YN1-2 located in themargin of the Tengchong Basin experienced a single ther-mal history during the late Miocene to Pliocene when theycooled at »> 110–70 °C during <7–0 Ma. Sample YN3located on the eastern side of the Gaoligong mountainsbelt, experienced a single thermal history during >40–33 Mawhen they cooled at »> 110–70 °C with very rapid coolingrates and followed by thermal stability during 30–0 Ma.

Fig. 4. Distributions of volcanic eruptions prior to the Pleistocene in the Tengchong volcanic Weld. In the Wgure, data represent K–Ar ages (Ma) of volcanicrocks. Data are cited from Mu et al. (1987), Liu et al. (1992), Nakai et al. (1993) and Li et al. (1999).

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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389 381

BrieXy, two distinct periods (»44–33 Ma, <»7.0 Ma) ofaccelerated cooling can be identiWed from modeled time-temperature cooling histories as shown above. These cool-ing events are the same time as yielded by Fan (2005) alongthe southern extension of the Gaoligong mountains belt.

4. Tectonic stress and earthquake focal mechanisms

Earthquakes in Tengchong and its adjacent areas(20–27°N, 90–100°E) since 1976 were collected from ISCcatalogue, and earthquake proWles with depth along longi-

tude (90–100°E) and latitude (20–26°N) were provided(Figs. 8 and 9-1). Earthquakes deeper than 40 km weremainly distributed from 93°E to 96°E, along the Burmasuture zone, that includes Naga Hills-Arakan Yoma suturezone in the eastern margin of the Indian plate and Myitky-ina-Mandalay suture zone. Between 92 and 94°E, thrustbelts have small oblique angles and extend to »100 km indepth, but in the area of 95°E, the thrust belts are near ver-tical to »180 km in depth. The earthquake distributionsbetween 95 and 97°E in the thrust belts were sparse, maybebecause the main collision belt changed from nearly NS- to

Fig. 5. Distributions of volcanic eruptions during the Pleistocene–Holocene in the Tengchong volcanic Weld. In the Wgure, data represent K–Ar ages (Ma)of volcanic rocks. Data are cited from Mu et al. (1987), Liu et al. (1992), Nakai et al. (1993) and Li et al. (1999).

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382 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

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NNE- trending at 24°N. However, to the east of 98°E,earthquakes occurred at 0–40 km in depth.

In the Tengchong volcanic Weld and its adjacent areas,shallow source earthquakes were developed extensively(Figs. 9-1 and 9-2). The Yunnan seismic network showsthat: (1) The activities of earthquakes in the Tengchongvolcanic Weld are weaker in frequency and intensity than itssurroundings; (2) There were no historical strong earth-quakes and the largest earthquakes recorded are Ms 5.0–5.8; (3) The microseismicity observations indicate thatmicroseismicity in the Tengchong volcanic Weld are atdepths of 1–6 km and are typical shallow source earth-quakes, but those outside the Weld are always deeper than20 km (Qin et al., 1996; Qin et al., 2000a).

Mid-source earthquakes occurring at 80–180 km depthsonly developed in Burma along the western side of the Sag-aing dextral strike-slip fault, which is located between 94–95.5°E. The source of the earthquakes is also the position ofthe Naga Hills suture zone, which was a subductional zoneduring 27–20 Ma where the Indian plate subducted underthe Burma–China micro-plate (Stephenson and Marshall,1984; Le Dain et al., 1984). Far from the subductional zone,focal depths are shallow. In the Tengchong and related vol-cano areas, the focal depth of earthquakes is »30–35 km,and no earthquake deeper than »40 km has been recorded.

In order to study the tectonic stress in the Tengchongarea, earthquake mechanisms of CMT solutions fromHarvard University were plotted (Fig. 9-2). The mecha-nism of shallow earthquakes shows that the main stress inBurma and Tengchong area is NE–SW horizontal com-pression. Based on seismic foci mechanisms and depthinformation of 660 earthquakes (Jiang et al., 1998), from98° to 100°E, the tectonic stress is complicated, with mainstress of NNE and NE, some NNW and NW, fewer ENEand ESE. But the main type of foci mechanism is shearing.Within a belt trending 20–24°N, distributions of earth-quakes are N–S-trending, and from 24°N on, the earth-quakes are distributed as limited in a NNE-trending belt(Figs. 8 and 10). The focal mechanism show the samecompressional stress. On the A–A� (N–S) seismic proWle(Fig. 10-a), the main compressional stress of 60% of theearthquakes are focused along a NNE–NE-trend, and ele-vation angles of P-axis for 89% of the earthquakes are<30°, which indicates that most of the earthquakes arecharacterized by horizontal shearing. A few earthquakesare characterized by oblique-slip displacement; theseearthquakes are primarily distributed along the Myitky-ina-Mandalay suture. On the B–B� (NE) seismic proWle(Fig. 10-b), the main compressional stress of 58% of theearthquakes are distributed along a NNE–NE-trend, and83% of the earthquakes yield elevation angles of P-axis of<30°. Most earthquakes, whose focal depths are shallowerthan 80 km, are characterized by horizontal shearing,while those earthquakes deeper than 80 km appear asthrust oblique-slip displacement. This indicates that alongthis section the regional tectonic stress Weld is created by aNNE–NE-trending horizontal compressional stress, but

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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389 383

deep earthquakes show the function of compressionalstress with larger elevation angle. On the C–C� (E–W)seismic proWle (Fig. 10-c), the main compressional stressof 64% of the earthquakes are focused along a NNE–NE-direction, and 87% of the earthquakes yield elevationangles of P-axis of <30°, similar to A–A� and B–B� pro-Wles. The main extensional stress axis of 55% earthquakesis focused along the NNW–NW direction. The earth-quakes shallower than 30 km are characterized by hori-zontal shearing displacement, and with the increasingdepth, earthquakes have larger strike-slip component. Allof the mechanisms of eight earthquakes deeper than120 km express transpressional displacement. On theNNW, NNE and almost NS-trending fault planes, the

dip-angle is larger than 60°, and appears as right-handhorizontal shearing displacement. Overall, earthquakes atshallow structural levels are characterized by horizontalshearing displacement while at deep structural levels,earthquakes are characterized by thrust oblique-slip dis-placement.

The earthquake depth distributions and focal mecha-nism in the Tengchong area are the same as those inBurma (Molnar and Tapponnier, 1978; Le Dain et al.,1984; Wang and Long, 2000). Based on the character ofearthquake mechanisms at diVerent structural depths, theIndian plate may produce lateral compression-shearing(transpression) and oblique thrust in the Burma suture atthe same time, so the compressional stress of earthquakes

Fig. 6. Apatite Wssion-track data plots.

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384 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

occurring in the crust are nearer to N–S-direction, andwith smaller elevation angles.

5. Discussion of tectonic controls on the Tengchong volcanic Weld

To characterize the tectonic setting of the Tengchongvolcanic Weld, we must address three questions: (1) How didthe tectonic setting constrain the locations of volcanic erup-tions and result in volcanic materials similar to those at anactive continental margin? (2) Was there subduction of theIndian plate under Burma or Tengchong block during thevolcanic eruptions? (3) Did northward penetration of theIndia Plate into Asia and resultant shear deformation con-tribute to generation of magmas and volcanism?

5.1. Magma source of volcanic eruptions in the Tengchong volcanic Weld

Calc-akaline basalt and shoshonite volcanic rocks aretypically associated with continental margin and orogenicbelt melting (Barr and Macdonald, 1981; Gill, 1981; Glaz-ner and Bartley, 1994). Based on petrologic and geochemi-cal data from the calc-alkaline basalt in the Tengchongarea, there have arisen diVerent viewpoints for their tec-tonic setting, including collisional zone between Indian andEurasian plates (Zhu et al., 1983; Mu et al., 1987; Jianget al., 1998), post-collision island-arc environment (Zhaoand Chen, 1992), or mature island-arc environment (Conget al., 1994). Zhu et al. (1983) suggested that a signiWcantproportion of subducted oceanic materials exist within the

Fig. 7. Thermal modeling of apatite Wssion-track length and age data Modeled time-temperature paths for three apatite samples, computed with AFTsolveprogram by Rich Ketcham and Raymond Donelick (2001) (version 1.3.0). Initial track length of 14.5 �m was used in constructing these models (Ketcham andDonelick, 2001). Temperature ranges between 110 and 60°C delineate apatite partial annealing zone, deWned as temperature interval in which majority of tracklength shortening take place. Additionally, several conditions are as follows: (1) the present day temperature is set to a constant value of 20 °C; (2) kinetic var-iable was DparD 1.50; (3) modeling scheme was Monte Carlo. We modeled 10,000 paths for each plot. Thick lines show best-Wt solutions obtained for thesemodel run, and dark-grey and light-grey colors show general-Wt solutions and good-Wt solutions obtained for same model run, respectively.

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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389 385

mantle beneath the Tengchong area, and in a back-arcextensional environment, these mantle-derived magmaswere erupted on the surface along the extensional fractures.However, the Tengchong block was in a continentalinterior, far away from the suture zone, during the Mio-cene-Quaternary. Ji (1998) suggested that the late Miocene–Pliocene volcanism in Tengchong area was related tolithospheric denudation. Yet, Fan et al. (1999) concludethat the origin of the volcanic magma was related to crustand mantle interaction; these Pliocene–Pleistocene volcanicrocks preserve evidence that crustal materials were assimi-lated into the magma (Fan et al., 1999).

The subduction of the Indian plate under the Eurasianplate occurred before »27–15 Ma, yet the Tengchong andBurma volcanoes erupted after »5.5 Ma. In the Gaoligongmetamorphic belt and Tengchong area, the sinistral strike-slip movement might also have been developed during »27-

15 Ma. Apatite Wssion track ages of »6–5 Ma indicate thatthe Gaoligong metamorphic belt was rapidly exhumed,which is somewhat earlier than eruptions within the Teng-chong volcanic Weld, but the same age as formation ofextensive basins in the Tengchong area (Figs. 11 and 12).

During the middle-late Cenozoic time, in the Tengchongarea and its surroundings, the volcanic eruptions of theTengchong volcanic Weld had not been controlled by sub-duction or island-arc environment. The volcanic eruptionsin the region occurred with similar rare-element distribu-tion models indicating that the diVerent stages of volcaniceruptions have a similar origin, and were from the samesource (Fan et al., 1999; Chen et al., 2002). Pb, Sr, and Ndisotope compositions prove that the source material of theTengchong volcanic rocks is EM II-type mantle, or old sub-duction and re-cycled mantle (Fan et al., 1999). This is simi-lar to the Cenozoic volcanic rocks erupted in the northern

Fig. 8. Tectonic sites of seismic proWles across the Tengchong volcanic Weld. Locations of A–A�, B–B� and C–C� in Fig. 10 are shown.

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386 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

Tibetan plateau and the Kunlun Mountains belt (XizangBureau of Geology and Mineral Resources, 1993; Dengand Sun, 1999; Lai, 1999; Wang, 1999).

The characteristics of the seismic source distributionsshow that oblique subduction is focused on the upper of100 km. The Tengchong volcanic Weld is far from Myitky-ina-Mandalay suture zone (about 120 km), and 450 kmfrom Indian plate (Figs. 1 and 12). When the Tengchongvolcanic Weld began eruptions during the end of Miocene–Pliocene, the Myitkyina-Mandalay suture had been closed.

So, the Tengchong volcanic rocks are the product of intra-continental magmatism, not a continental margin or colli-sional magmatism between Indian and Eurasian plates.

5.2. Sagaing dextral strike–slip fault and probable constraints to the Tengchong volcanic Weld

Naga Hills subduction in the Burma arc initiated in theOligocene to Miocene and ceased during Miocene time(Stephenson and Marshall, 1984). During subduction,

Fig. 9-1 and 9-2. 9-1 Seismic distributions in the Tengchong volcanic Weld and surrounding areas and 9-2 earthquakes beneath the Tengchong area. Loca-tion of seismic proWle is same as section C–C� in Fig. 10.

0

30

60

90

120

150

180

210

90 92 94 96 98 100

Longitude (˚)

Dep

th (

km)

0306090

120150180210

20 22 24 26

Latitude (˚)

Dep

th (

km)

05

101520253035

98 98.2 98.4 98.6 98.8 99

Longitude (˚)

Dep

th (k

m)

Fig. 10. Seismic proWles in diVerent orientation (A–A�, B–B� and C–C�) and their foci mechanism (simpliWed from Wang and Long, 1998).

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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389 387

volcanism was absent. Since the middle-late Miocene toPliocene, »460 km displacement accumulated along dex-tral strike-slip of the Sagaing fault. On both sides of theSagaing fault zone, the Burma and Tengchong volcanicWelds developed at the same time and are characterized bysimilar compositions (Zhu et al., 1983; Le Dain et al.,1984; Stephenson and Marshall, 1984; Whitford-Stark,1987). Besides the volcanic characteristics and regionaltectonic stress Weld, the seismic mechanism in the Burmavolcanic area appears as near N–S-direction compressionon P-axis, which is similar to that in the Tengchong area(Fig. 9-1 and 9-2). Sagaing dextral strike-slip fault motionalso occurred at the same time as extension of the basinsin the Tengchong area. An east-dipping inclined zone ofintermediate depth earthquakes suggest that a slab of oce-anic lithosphere was subducted to the east under theIndoburma ranges (Le Dain et al., 1984). The Sagaingfault probably accommodates most of the right-lateralslip of India past Indochina, but large scale northward

movement of the Indian plate also causes internal defor-mation within Burma, Tailand and Yunnan Province inChina (Le Dain et al., 1984).

From the tectonic stress in the Tengchong volcanicWeld, there is a NNE–NE direction horizontal shearingand compression. Northward motion of the Indian plateresults in drag along its eastern margin resulting in thetransformation of the Myitkyina-Mandalay plate sutureto a transtension zone. This drag also results in the gener-ation of the dextral Sagaing fault. The Sagaing dextralstrike-slip movement resulted in and has controlled theformation and activation of the Tengchong volcanic erup-tions since the late Miocene–Pliocene. This tectonicregime also resulted in reversal of slip along W-dippingthrusting faults that they exhibit normal slip and genera-tion of new normal faults in the Tengchong volcanic Weld.These normal faults and associated fractures providepathways for the magma upwelling and eruption on thesurface. Therefore, we conclude that shearing motion

Fig. 11. Composite diagram of structures, tectonic stress, volcanic eruptions and timing sequences of the Tengchong volcanic Weld along the southeasternmargin of the Tibetan plateau.

Fig. 12. Tectonic Model of origin and formation of the Tengchong volcanism along the southeastern margin of the Tibetan plateau.

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388 Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389

along the eastern margin of the Indian plate lead to thevolcanic activation.

Continental margin volcanic rocks did not form duringthe middle-early Cenozoic subduction of the Indian plateunder the Eurasian plate. From the structures and tec-tonic stress changes and regional structural evolution(Fig. 11), we suggest that the magma of the Tengchongarea was generated during the formation of a collisionalorogenic belt, but eruption of the magma was controlledby structures developed during transtensional deforma-tion (Fig. 12). The formation of the normal faults anddextral strike-slip faults was a consequence of northwardmotion of the Indian plate. The volcanic eruptions wereprogressively younger from south to north, which seemsto imply that they are correlated to the northward motionof the Sagaing fault.

6. Conclusions

In the Tengchong volcanic Weld, eruptions of the high-K calc-alkaline basalt series and andesite series volcanicrocks occurred during the late Miocene to Holocene. Atleast four stages of volcanic eruptions have been classiWedbased on isotopic ages: the oldest stage is » 5.5–4.0 Ma,the next stage is 3.9–0.9 Ma, the penultimate stage is 0.8–0.1 Ma, and the youngest stage is <0.01 Ma. NS-NNE-striking with E- or WNW-dipping regional normal faultsand dextral strike-slip faults were developed. The Teng-chong area and the Burma volcanic arc share similar pet-rological and geochemical characteristics, and eruptiontime sequences. The geochemical and petrologic evidencesuggests that the magma was generated in the former sub-duction zone which developed before »27-15 Ma. Sincethe late Miocene–Pleistocene, transtensional dextralstrike-slip motion along the Sagaing fault resulted in asmall component of EW-extension resulting in magmaeruption. These motions mainly resulted from the contin-uous northward penetration of India plate to Eurasia.Based on the time of formation of faults and basins, anal-ysis of the cooling history of the mountain belt, as wellcalculations of seismic foci mechanism, the Sagaing faultdextral strike-slip motion is a major factor that constrainsthe deformation and volcanic eruptions in the Tengchongvolcanic Weld. The volcanic eruptions from the late Mio-cene were not in a continental margin but rather in anintracontinental tectonic setting.

Acknowledgements

This research is supported by a Main Project (95-11-03)from the China Seismological Bureau and the ChinaNational Basic Research Program Project (2002CB412601).Discussions with Profs. Wang, C.Y., Fan, Q.C. and Wang,E.C. help to improve the manuscript. We are indebted toProf. J.L. Whitford-Stark and an anonymous reviewer fortheir constructive comments and suggestions, and Dr.JeVrey Lee for improvements to English.

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